Geologic Map of the Helvetia 7.5' Quadrangle, Pima County, Arizona. One sheet, map scale 1:24,000.

The Santa Catalina and Rincon Mountains of southeastern Arizona (Fig. 1) were seminal in the development of the concept of metamorphic core complexes (Davis and Coney, 1979). The first steps in this development were the recognition and description of low-angle normal faults (Pashley, 1966) and mylonitic rocks (Ted Theodore and Norm Banks in Banks, 1974) all along the sinuous range fronts (Fig. 1). Because early workers in these areas noted a striking parallelism and identical vergence of mylonitic fabrics and structurally overlying detachment faults (Davis, 1980; Wust, 1986; Reynolds and Lister, 1990), these two features became married as deep ductile and shallow brittle aspects, respectively, of extensional tectonics (e.g., Spencer and Reynolds, 1989). The Catalina-Rincon metamorphic core complex became a "type" example partly by virtue of its early description.

During the period April IS-October 18, 1996, the authors mapped the geology of about 110 square kilometers west of Nogales, Sonora, Mexico along the international border near Los Adobes Rancho for Minefinders Corporation, Ltd .. Approximately 46 days were spent in the field. Reconnaissance geologic mapping focused primarily on the Mesozoic and Tertiary igneous and sedimentary rocks that lie both north and south of the main road into the area. No fossil or radiometric age data were obtained from the area, and age assignments, made on stratigraphic and intrusive relationships, are provisional at best.

The accompanying geologic map (scale 1 :24,000) displays the structural and stratigraphic relationships of faulted homoclines of largely undated mid-Tertiary strata exposed over an area of ~250 km2 in the drainages of Camp Grant, Putnam, Dodson, and James Washes in uplands between the Santa Catalina and Tortilla Mountains west of the San Pedro trough. The dominantly homoclinal mid-Tertiary strata dip generally east-northeast, and are cut by normal faults dipping generally west-southwest. Previous maps of various parts of the study area at comparable scale (Krieger, 1968, 1974a, 1974b, 1974c; Hansen, 1983) were prepared without information now available about regional stratigraphic relationships (Dickinson, 1991). Most key geologic features were plotted correctly during previous mapping at smaller scale (Dickinson, 1987, sheets 3, 4, 6; 1988; 1991, figs. 1, 46; 1992), but the detailed placement of contacts and structures on the accompanying geologic map supersedes past mapping. In particular, an exposure of the Cloudburst Formation is delineated between James and Polecat Washes near the northern end of the map area, the position of the base of the Quiburis Formation along the western flank of the San Pedro trough is revised (from Dickinson, 1998, sheet 2) in the vicinity of Dodson and James Washes to the north of Elgin Wash, and transverse east-west faults (Cactus Hill and Putnam Wash faults) are mapped more completely and accurately than before.

Past geologic maps and accounts of the sedimentary, geomorphic, and structural geology of the piedmont of the Santa Catalina Mountains in the outskirts of Tucson by Voelger (1953), Pashley (1966), Davidson (1973), Creasey and Theodore (1975), Banks (1976), Anderson (1987), Pearthree et al. (1988), McKittrick (1988), and Jackson (1989) raised questions about the sedimentary evolution of the foothills belt that were left open or unresolved. Multiple dissected alluvial fans and pediments overlie much older faulted and tilted strata to form Cenozoic sedimentary assemblages of considerable stratigraphic and structural complexity. As a contribution to improved understanding of urban geology in the Tucson metropolitan area, systematic geologic mapping of the piedmont belt was undertaken to establish the overall geologic framework of the Catalina foothills (~ 175 sq km) at a common scale (1 :24,000). The area studied extends from Oracle Road on the west to the vicinity of Rinconado Road on the east, and from the Rillito River and Tanque Verde Wash northward to the limit of bedrock exposures at the base of the Santa Catalina Mountains.

The geologic map (at a scale of 1 :24,000) depicts the full extent of the Ripsey Wash compound half-graben, which occupies an elongate area of approximately 15 sq km parallel to the Ripsey fault, and represents a revision and update of previous mapping (Schmidt, 1971; Cornwall and Krieger, 1975a, 1975b; Dickinson, 1991, Figure 40). Access to the interior of the study area, which lies within the Grayback and Kearny 7.5 minute USGS quadrangles, is provided by an unimproved road that turns up the floor of Ripsey Wash from the graded Florence-Kelvin highway, which crosses Ripsey Wash just above its mouth near the A-Diamond Ranch. The regional geologic relationships of the Ripsey Wash half-graben are discussed elsewhere (Dickinson, 1991, 1995).

The Pirate fault is a basin-range normal fault that separates footwall bedrock of the Santa Catalina Mountains on the east from downfaulted Neogene basin fill underlying Oro Valley, which forms a sedimented corridor to the west between the Santa Catalina and Tortolita Mountains (Pearthree and others, 1988). According to both McCullough (1963) and Budden (1975), this basin-margin, range-front structure was first termed the Pirate fault by Wallace (1954), although his own thesis makes no explicit claim for originating the nomenclature. The trend of the fault is northeasterly at the foot of Pusch Ridge near Tucson, but swings to nearly due north along the range front north of Catalina State Park. In detail, the structure is a complex fault zone, with brecciation and complex fracturing of varying intensity present for 100-200 m into footwall bedrock (Davis and others, 1994). By contrast, offset basin fill forming the hanging wall is little deformed, at least at exposed horizons. The contact between faulted basin fill and variably deformed bedrock is a thin gouge seam, only 1-5 cm thick, that dips consistently at angles of 50-55 degrees beneath Oro Valley. This contact is here inferred to be the main displacement surface of the Pirate fault zone, and its trace is the feature plotted along the trend of the fault. Adjacent to the gouge seam in the footwall is a cohesive sheet of compact fault breccia, composed of crushed bedrock and typically 4 m thick (Davis and others, 1994).

The geologic map (at a scale of 1:24,000 or two inches equals 2000 feet), covering an area of approximately 35 sq km between the Willow Springs Ranch on the south and Star Flat on the north, is a revision and update of Krieger (1974a, 1974b), who showed only the northern fringe of the study area, and Dickinson (1987, 1992), who depicted the geologic relations at smaller scale (1,62,500 and 1:125,000, respectively). Access to the study area is provided by Willow Springs Road, which turns north off the paved highway between Oracle and Oracle Junction. The regional geologic context ofthe Star Flat-Willow Springs area is discussed by Dickinson (1991).

Geologic Map of the Sheep Basin Mountain Area, Northern Sierra Anchas, Gila County, Arizona. Map scale 1:10,000, 1 map sheet, mapping by Timothy B. Brady 1986-7 on USGS Topographic Base (Sheep Basin Mountain 1:24,000).

The text and geologic maps that compose Bulletin 198 represent a major advance in our understanding of the geology and mineral resources of west-central Arizona. The Buckskin and Rawhide Mountains, along with the adjacent Harcuvar and Harquahala Mountains, are one of the approximately 25 metamorphic core complexes in western North America. The geology of the Buckskin and Rawhide Mountains is dominated by the BuckskinRawhide detachment fault, a subhorizontal, undulating normal fault. Detachment faults are characteristic features of metamorphic core complexes. Improved understanding of detachment faults is critical in formulating more accurate models of the genesis of mineral deposits in Arizona. 279 p.